As the rate of data transfer between microelectronic devices increases, use of typical electrical bus schemes to transmit information becomes increasingly problematic. In particular, as the amount of information transfer increases, an amount of input/output power required to transmit information between devices and consequently an amount of electronic noise associated with the transmission increase.
Another problem associated with transmission of electrical signals using traditional electrical bus systems is that signal attenuation and distortion increases as the rate of the transmitted signal increases. For example, when signals are transmitted at a rate of about 5 GHz using FR-4 substrate material, the signal suffers about a 5 dB loss across 10 cm. This loss can cause rise time degradation and amplitude loss for the signals as the higher order harmonics are filtered out. For high data rate transmission across greater lengths, potentially up to several kilometers, optical transmission is required. Accordingly, improved apparatus and systems for transmitting information between a plurality of microelectronic devices optically are desired. For the above reasons, although the majority of signal processing is done in the electrical domain, it has become highly advantageous to utilize optical communications to interconnect microelectronic devices.
In order to obtain the benefits of optical communications, electrical signals must be converted to optical signals and vice versa. Various high powered lasers have been developed in the telecommunication art for transmitting optical signals through optical fibers over long distances, such as many miles. Short to medium distance optical communications, such as within one box, or through optical cables that are not more than a few hundred meters in length, require various other light emitting devices. For such applications, a variety of lasers including Fabry-Perot and vertical cavity surface emitting lasers (VCSELs) have been developed. However, specialized circuits are needed to advantageously utilize this technology. In particular, such lasers require specialized electronic current drive circuits in order to modulate light output. There is a need in the art for improvements in such specialized circuits. What is desired is a precisely controlled true current source to achieve relative insensitivity to the characteristics of the various light emitting devices. There is also a need for such improved circuits to provide temperature compensation and efficient over voltage protection.